Keywords: high speed line, vertical track stiffness, maintenance costs, ballasted track, slab track, infrastructure design.

In the railway field the existence of zones with abrupt changes in the vertical track stiffness all along a railway line are well known. This requires the need for constant maintenance activities to maintain the quality of the track within an acceptable range which, in turn causes an increase in the operating and maintenance costs of the line. The causes of the existence of these zones are associated with certain features related to the superstructure and infrastructure.

In a railway line one can find, in close proximity, sections with the same (or similar) superstructure but different infrastructure characteristics. For example different embankment heights, different materials that made up the embankment, different formation layer thicknesses or materials or different natural ground. Similarity, one also can find sections with a variation in the superstructure. For example in embankment-tunnel transition where the track changes from ballasted to slab. This may happen with rough topography, forcing the construction of many tunnels and structures separated by some sections of embankments of varying height. This fact is even more common in high speed lines where the design parameters are more restrictive.

This longitudinal variation of the type of superstructure and infrastructures turns into the appearance of longitudinal heterogeneity of the vertical track stiffness value. To deal with this problem, no matter what the cause of its variation, new design criteria are proposed in this paper, focusing on different infrastructure components.

A three-dimensional finite element numerical model of a section of the railway track was developed in order to analyse the behaviour of both superstructure and infrastructure to the passage of railway loads. This model has been configured to enable the calculation and review of several geometries that define the cross section and the geotechnical characteristics of its different materials. In addition, the vertical track stiffness value can be obtained, allowing the definition of design criteria associated with the stiffness value to be attained.

The proposed model, and hence the new criteria, was applied to a real Spanish high speed rail line under construction between Lemoa and Galdakao in País Vasco. This line goes through an area of difficult topography and about 70% of its proposed length is in tunnels or on bridges. This characteristic is used to show the advantages and applicability of the proposed new criteria in the cross section design in order to achieve a better homogeneity in the vertical track stiffness value and, therefore, to reduce the consequences (from a technical and economical point of view) of this variation on the operation of the line.

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